The present invention relates to a gas laser oscillation apparatus, and, more particularly, to an improvement in a structure for mounting various mirrors, of a laser resonator, on a mirror supporting frame.
Generally, in a gas laser oscillation apparatus, a gaseous laser medium is excited by glow discharge to generate a laser beam.
More particularly, in a high power type gas laser oscillation apparatus, the temperature of the apparatus is remarkably raised due to the glow discharge. In order to minimize the adverse effect of the above-mentioned temperature rise, a laser tube, forming a laser resonator, is mounted on a base in an expansible or contractible manner, and mirrors, connected to the laser tube at both ends and a part thereof, are mounted on a mirror supporting frame movable in a direction of the axis of the laser tube.
Further, a Japanese Patent Application (Publication No. 3920/1975) discloses adjusting means for finely displacing a spherical mirror in alignment with an optical axis.
As shown in FIG. 1, a known gas laser oscillation apparatus includes four laser tubes 1, namely, glass tubes, connected optically in series to form a laser tube connection body. A gaseous laser medium is enclosed in each sealed laser tube 1, and a cathode 2 and an anode 3 are arranged at both ends of each laser tube 1. A high voltage from a d.c. high voltage source 4 is applied to the cathode 2 of each laser tube 1 to generate glow discharge in the laser tube 1.
A total reflection mirror 5 and an output mirror 8 are mounted on both ends of the laser tube connection body, and a hollow block member 6, provided with a pair of mirrors for bouncing a laser beam back and forth, is included in the laser tube connection body at a middle part thereof. The laser tube connection body, total reflection mirror 5, output mirror 8, and block member 6 form a laser resonator.
The laser resonator is mounted through holding parts 12 on a base 13 in such a manner that the resonator can expand or contract in the direction of the axis of laser tube 1. On the other hand, a mirror supporting frame generally designated by the reference numeral 10 is mounted through the holding parts 12 on the base 13 in such a manner that the frame 10 is placed in substantially the same plane as the laser tube connection body. The mirror supporting frame 10 includes a pair of longitudinal frames 10A disposed in parallel to the laser tube 1 and a pair of transverse frames 10B disposed perpendicular to the laser tubes 1, to form a rectangular frame.
The total reflection mirror 5 and the output mirror 8 are mounted directly on one of the transverse frames 10B, and the block memeber 6 is mounted directly on the other transverse frame 10B. Metal bellows 28 and the like are included in the laser tube connection body in order to absorb the thermal deformation of laser tube 1 in the axial direction.
The gaseous laser medium is excited by the glow discharge in each laser tube 1 to generate a laser beam. The laser beam is generated back and forth between the total reflection mirror 5 and the output mirror 8, formed as a partial reflection mirror 11, through a pair of mirrors provided in the block member 6, and is thereby amplified. A part of the laser beam thus amplified passes through the output mirror 8, and is emitted from the laser resonator as an output beam 9.
In order to prevent the total reflection mirror 5, output mirror 8 and a pair of mirrors 11 from being thermally displaced due to a temperature rise caused by the glow discharge in each laser tube 1, these mirrors are mounted on the mirror supporting frame 10 and supported by the base 13 in such a manner that these mirrors can freely move in the direction of the optical axis of the laser tube connection body.
Further, the pressure of the gaseous laser medium in each laser tube 1 is on the order of 4 kPa (kilopascals), and, can be regarded as a vacuum. Accordingly, a large vacuum force F is applied to the mirrors, namely, the total reflection mirror 5, the output mirror 8 and a pair of mirrors 11, which serve as walls between vacuum and atmospheric pressure on the order of 100 kPa. For example, when the total reflection mirror 5 and the output mirror 8 each mounted on the transverse frame 10B of the mirror supporting frame 10 have a diameter of 10 cm, a vacuum force F of about 80 KgW is applied to each of these mirrors.
FIGS. 2 and 3 show the bending of the mirror supporting frame 10 shown in FIG. 1 due to the abovementioned vacuum force, and the deviation of the center axis of each mirror caused by the above deflection.
In the case where the transverse frame 10B is formed of a steel plate having a length L of 100 cm, a thickness t of 1 cm and a width W of 20 cm, and the total reflection mirror 5 and the output mirror 8 are mounted on the transverse frame 10B at positions which are spaced apart from both ends of the frame 10B by a length l.sub.1 of 30 cm, the transverse frame 10B is deformed as indicated by dotted lines in FIGS. 2 and 3, due to the vacuum force of about 80 KgW applied to the mirrors 5 and 8, and a maximum deflection V at a central part of the frame 10B is nearly equal to 20 .mu.m.
Due to such deformation of the transverse frame 10B, the center axis of each of the total reflection mirror 5 and the output mirror 8 is inclined at an angle .theta. of about 4.times.10.sup.-5 rad with the optical axis X of the laser resonator, as shown in FIG. 3. As a result, when the total length of the laser resonator is equal to 6 m, the optical axis of the laser resonator is displaced on the mirrors 5 and 8 by about 2 mm each time the laser beam makes one round trip between the mirrors 5 and 8. Such displacement of the optical axis decreases the power of the output beam.
As mentioned above, the conventional laser oscillation apparatus shown in FIGS. 1 to 3, namely, the laser oscillation apparatus in which the mirrors 5, 8 and 11 are mounted directly on the mirror supporting frame 10, has a drawback that the center axis of each of these mirrors 5, 8, 11 deviates from the optical axis of the laser resonator due to the deformation of the mirror supporting frame caused by the vacuum force F. In a laser apparatus, in which a high-accuracy adjustment of optical axis is required, the above-mentioned drawback is fatal, and therefore the power of output beam is decreased remarkably. Further, in a case where the laser tube connection body is large-sized, that is, a diameter of each mirror and the length of the laser resonator are larger in order to obtain a high-power laser apparatus, the deformation of the mirror supporting frame is increased, the deviation of the center axis of each mirror from the optical axis is also increased, and finally it becomes impossible to generate laser oscillation.